There has been an increasing interest in the development of laser sources that emit in the mid-infrared (“mid-IR”) spectral region, i.e., at wavelengths between about 2.5 and 8 μm. Such lasers have significant uses for both military and non-military applications. In the military realm, mid-IR lasers can be extremely useful as a countermeasure to jam heat-seeking missiles and prevent them from reaching their targets. In both the military and non-military realm, such mid-IR lasers have found use, for example, in chemical sensing, and so may be very useful in environmental, medical, and national security applications.
On the short-wavelength side of this spectral region, type-I quantum-well antimonide lasers are achieving excellent performance and greater maturity. See, e.g., L. Shterengas et al., “Continuous wave operation of diode lasers at 3.36 μm at 12° C.,” Appl. Phys. Lett. 93, 011103 (2008). On the long-wavelength side of the mid-IR, intersubband quantum cascade lasers (QCLs) have become the dominant source of laser emissions. See, e.g., S. Slivken, et al., Compound Semiconductors (October 2008), at p. 21.
For the mid-infrared spectral region, the interband cascade laser (ICL) is being developed as a promising semiconductor coherent source.
The first ICLs were developed by Rui Yang in 1994. See U.S. Pat. No. 5,588,015 to Yang. The ICL may be viewed as a hybrid structure, which resembles a conventional diode laser in that photons are generated via the radiative recombination of an electron and a hole. However, it also resembles a quantum cascade laser in that multiple stages are stacked as a staircase, such that a single injected electron can produce an additional photon at each step of the staircase. See Slivken et al., supra; see also U.S. Pat. No. 5,457,709 to Capasso et al. This photon cascade is accomplished by applying a sufficient voltage to lower each successive stage of the cascade by at least one photon energy, and allowing the electron to flow via an injector region into the next stage after it emits a photon. An interband active transition requires that electrons occupying states in the valence band (following the photon emission) be reinjected into the conduction band at a boundary with semi-metallic or near-semi-metallic overlap between the conduction and valence bands. Outside of the active quantum well region and hole injector, transport in the ICL typically takes place entirely via the movement of electrons, although this is not required. Therefore, two optical cladding regions are generally used at the outsides of the gain medium to confine the lasing mode along the injection axis, and n-type contacts are provided outside the cladding regions to provide for electrical bias and current injection.
ICLs also employ interband active transitions just as conventional semiconductor lasers do. Although type-I ICLs are also possible (see U.S. Pat. No. 5,799,026 to Meyer et al., two inventors of which are the inventors of the present invention, and which is incorporated by reference into the present disclosure), most ICLs employ active transitions that are of type-II nature, i.e., the electron and hole wavefunctions peak in adjacent electron (typically InAs) and hole (typically Ga(In)Sb) quantum wells, respectively. In order to increase the wavefunction overlap, two InAs electron wells often are placed on both sides of the Ga(In)Sb hole well, and create a so-called “W” structure. In addition, barriers (typically Al(GaInAs)Sb) having large conduction- and valence-band offsets can surround the “W” structure in order to provide good confinement of both carrier types. See U.S. Pat. No. 5,793,787 to Meyer et al., which shares an inventor in common with the present invention and which is incorporated by reference herein. Further improvements to the basic ICL structure, such as including more than one hole well to form a hole injector, were subsequently made by the present NRL inventors and Dr. Yang. See U.S. Pat. No. 5,799,026 to Meyer et al., supra.
Despite these improvements, the performance of the first ICLs fell far short of the theoretical expectations. In particular, the threshold current densities at elevated temperatures were quite high (5-10 kA/cm2 at room temperature in pulsed mode), which precluded continuous-wave (cw) operation of those devices at temperatures higher than ≈150 K. Later work by researchers at Army Research Laboratory, Maxion, and Jet Propulsion Laboratory further improved the operation of ICLs. See e.g., R. Q. Yang et al., “High-power interband cascade lasers with quantum efficiency>450%,” Electron. Lett. 35, 1254 (1999); R. Q. Yang, et al., “Mid-Infrared Type-II Interband Cascade Lasers,” IEEE J. Quant. Electron. 38, 559 (2002); and K. Mansour et al., “Mid-infrared interband cascade lasers at thermoelectric cooler temperatures,” Electron. Lett. 42, 1034 (2006). However, performance in the mid-IR at room temperature still remained unsatisfactory.